The scanner is an extension for 0 1 1 the ‘All-solid-state preamplifier’ published in Elektor Electronics* As its name implies, it scans all inputs of the preamplifier to ascertain where there is an audio signal present. That input re-mains selected. After there has been no signal for some time, scanning is resumed. The scanning action is provided by rectan-gular-wave generator IC2e.The output of this oscillator is applied to one of the input selector keys via buffer/inverter IC2d and diode D3. The diode prevents the key being disabled when the oscillator is off. The oscillator is switched on and off by IC2b, which in turn is controlled by IC lb. That opamp is configured as a comparator whose voltage threshold is preset with P1. The inputs of the scanner are linked to the audio inputs on the volume control board of the preamplifier. The relevant signal is amplified x40 by ICi, and IC id, after which summing amplifier ICia combines the left-1 hand and right-hand signals. As soon as
there is music or speech on the input lines, C1 will partly discharge rapidly. when the voltage across the capacitor drops below the level set by P1, ICib disables the oscilla-tor via IC2b and the input selected at that instant remains actuated. As long as there is a signal coming in, part of the charge on C1 will ebb away via ICia. If there is no sig-nal for some time (presettable with P2 between 3 s and 25 s), the capacitor will be charged almost completely via P23, R8 and D1. Once the terminal voltage of C1 rises above the comparator threshold, the oscil-lator is enabled again and the scanning action resumes. A total scan of all inputs is completed in 3 s (determined by R11-C2). The input sensitivity can be set between 10 mV and 4 V with Fel. The current drawn by the scanner is not greater than 10 mA.